1. Field of the Invention
The present invention relates to a surface light emitting semiconductor laser element and a method of manufacture thereof. More particularly, the present invention relates to a surface light emitting semiconductor laser element and a method of emitting laser light in a single-peak transverse mode.
2. Description of the Related Art
A surface light emitting semiconductor laser element emits laser light in a direction orthogonal to a surface of a substrate, and is a remarkable light source for application in various fields.
The surface light emitting semiconductor laser element has a semiconductor substrate, a pair of upper and lower reflectors, i.e., Diffractive Bragg Reflectors (DBRs) comprising compound semiconductors having different refractive indices on the substrate, and an active layer that constitutes a light emitting area between the pair of reflectors.
Typically, the surface light emitting semiconductor laser element has a post-type mesa structure were the upper DBR has a current confinement area. For example, Japanese Unexamined Patent Application Publication No. 2001-210908 discloses a surface light emitting semiconductor laser element comprising a circular post-type mesa structure having a mesa diameter of about 30 μm obtained by dry etching the upper DBR, and a current confinement area within the circular post-type mesa structure formed by selectively oxidizing an AlAs layer to efficiently inject current into the active layer.
Referring to the above-mentioned Japanese patent application publication and FIG. 12, a conventional surface light emitting semiconductor laser element comprising a post-type mesa structure will be described. FIG. 12 is a sectional view showing the structure of the conventional surface light emitting semiconductor laser element disclosed in the above-mentioned patent application publication.
As shown in FIG. 12, a surface light emitting semiconductor laser element 80 has a laminated structure sequentially comprising an n-type GaAs substrate 82, a lower diffractive bragg reflector (hereinafter “lower DBR”) 84 comprising an n-type semiconductor multi-layer, a lower clad layer 86 comprising non-doped AlGaAs, a light emitting layer (active layer) 88, an upper clad layer 90 comprising non-doped AlGaAs, an upper diffractive bragg reflector (hereinafter “upper DBR”) 92 comprising non-doped AlGaAs, and a p-type GaAs cap layer 94.
The lower DBR 84 has a semiconductor multi-layer structure including 30.5 pairs of n-type Al0.2Ga0.8As layers and n-type Al0.9Ga0.1As layers having composition gradient layers on the hetero interfaces. The upper DBR 92 has a semiconductor multi-layer structure including 25 pairs of p-type Al0.2Ga0.8As layers and p-type Al0.9Ga0.1As layers having composition gradient layers on the hetero interfaces.
A cylindrical mesa post 96 is formed by etching the cap layer 94, the upper DBR 92, the upper clad layer 90, the active layer 88, the lower clad layer 86, and the lower DBR 84.
A p-type AlAs layer is formed instead of the p-type Al0.9Ga0.1As layer on the compound semiconductor layer of the upper DBR 92 at the nearest side of the active layer 88. Al contained in the p-type AlAs layer is selectively oxidized excluding a center circular area to provide an oxidized-Al current confinement layer 98.
The p-type AlAs layer remaining on the center circular area functions as a current injection area 98A, and the oxidized-Al current confinement layer functions as an insulation area 98B having high electrical resistance.
A SiNx film 100 is formed over the mesa post 96 and the lower DBR 84. The SiNx film has an opening for exposing the p-type GaAs cap layer 94 provided by circularly removing the SiNx film 100 on the upper surface of the mesa post 96. A circular p-side electrode (upper electrode) 102 is formed at the periphery of the opening. On the opposite surface of the n-type GaAs substrate 82, an n-side electrode (lower electrode) 104 is formed. The p-side electrode 102 has an extraction electrode 106.
Referring to FIGS. 13A and 13B, a method of producing the surface light emitting semiconductor laser element 80 will now be described. FIG. 13A and 13B are sectional views showing steps of producing the surface light emitting semiconductor laser element 80.
As shown in FIG. 13A, the laminated structure is formed by sequentially laminating the lower DBR 84, the lower clad layer 86 comprising non-doped AlGaAs, the active layer 88, the upper clad layer 90 comprising non-doped AlGaAs, the upper DBR 92, and the p-type GaAs cap layer 94 on the n-type GaAs substrate 82.
The lower DBR 84 is produced by laminating 30.5 pairs of the n-type Al0.2Ga0.8As layers and the n-type Al0.9Ga0.1As layers having the composition gradient layers on the hetero interfaces. The upper DBR 92 is produced by laminating 25 pairs of the p-type Al0.2Ga0.8As layers and the p-type Al0.9Ga0.1As layers having the composition gradient layers on the hetero interfaces.
Before the upper DBR 92 is formed, the p-type AlAs layer 108 is formed instead of the p-type Al0.9Ga0.1As layer on the compound semiconductor layer of the upper DBR 92 at the nearest side of, or adjacent to the active layer 88.
As shown in FIG. 13B, the p-type GaAs cap layer 94, the upper DBR 92, the AlAs layer 108, the upper clad layer 90, the active layer 88, and the lower clad layer 86 are partially etched using a SiNx film mask 110 until the upper surface of the lower DBR 84 is exposed, whereby a mesa post 96 is formed.
The laminated structure having the mesa post 96 is heated at 400° C. for about 25 minutes under steam atmosphere to selectively oxidize only the p-type AlAs layer from the side face to the center of the mesa post 96.
Thus, a current confinement layer 98 is formed. The current confinement layer 98 has the cylindrical current confinement area 98B comprising the oxidized-Al layer, and the circular current injection area 98A comprising the p-type AlAs layer 108 that is not oxidized and remains. The circular current injection area 98A is surrounded by the current confinement area 98B.
After the SiNx film 100 is formed over the entire surface, the SiNx film 100 on the upper surface of the mesa post 96 is circularly removed to expose the p-type GaAs cap layer 94 where the circular p-side electrode is formed. At the opposite surface of the n-type GaAs substrate 82, the n-side electrode 104 is formed. As a result, the conventional surface light emitting semiconductor laser element 80 is provided.
In the surface light emitting semiconductor laser element comprising the post-type mesa structure, the current confinement layer 98 defines a section of a path for injecting a current into the active layer 88. Therefore, the current is intensively injected into the active layer 88 around the current confinement area 98B, which leads to efficient laser oscillation.
Typically, the conventional surface light emitting semiconductor laser element oscillates in a multi-mode which is a transverse mode having a plurality of peaks in the far field pattern (FFP).
When the surface light emitting semiconductor laser element is lens-coupled to an optical waveguide such as an optical fiber in the communication field, the surface light emitting semiconductor laser element desirably emits beams in a single-peak transverse mode, i.e., a Gaussian distribution mode, in order to improve the optical connection efficiency.
In the oxidized-type current confinement structure, the number of modes in the oscillating laser light is substantially in proportion to the size of the current confinement layer. Therefore, when the current injection area in the current confinement layer is decreased, it is possible to emit light in a single mode excited in a narrow area of the active layer.
Accordingly, in the conventional surface light emitting semiconductor laser element having the oxidized-type current confinement structure, when the size of the current confinement structure (current injection area) comprising the oxidized-Al layer is reduced, the light-emitting area of the active layer can be decreased and light is selectively oscillated in the single-peak transverse mode.
In order to provide the single-peak transverse mode, the size of the current confinement structure should be as small as 4 μm or less, as reported in IEEE. Photon, Tech. Lett. Vol. 9, No. 10, p. 1304, by M. Grabherr et al. However, if the size of the current confinement structure is 4 μm or less, the following problems occur.
Firstly, the tolerance of production errors becomes limited, since the size of the current confinement structure is extremely small. It is difficult to produce a surface light-emitting semiconductor laser element having a current confinement structure with a small diameter with good controllability. Also, wafer in-plane uniformity becomes poor, resulting in significantly decreased yields.
Secondly, current flows through the decreased current injection area (AlAs layer) by one order of magnitude as compared with the typical devices, whereby the resistance of the element becomes high, i.e., 100Ω or more. As a result, the output as well as the current and light emission efficiencies are lowered. In other words, since the output depends on the single-peak transverse mode, it is difficult to provide high output from the surface light emitting semiconductor laser element in the single-peak transverse mode.
Thirdly, due to the increased resistance caused by the current confinement, the impedance is mismatched. If the surface light emitting semiconductor laser element is attempted to be driven at high frequency, the high frequency properties are significantly degraded. Accordingly, it is difficult to apply the surface light emitting semiconductor laser element to light transmission driven at high frequency, as required in the communication field.
For transverse mode control of the laser light in a surface light emitting semiconductor laser element, Japanese Unexamined Patent Application Publication No. 2002-359432 discloses, for example, a method of stabilizing the transverse mode by processing a light emitting surface. However, this publication is not directed to the stabilization of the single transverse mode, but to the stabilization of a higher-order transverse mode.
Japanese Unexamined Patent Application Publication No. 2001-24277 discloses that a reflectance distribution is provided at a reflecting surface opposite to a light-emitting surface to stabilize the transverse mode. However, since light is injected through a substrate, it is difficult to apply this invention to a surface light emitting semiconductor laser element. In addition, since a proton-injection-type is presumed, it is difficult to apply this invention to the oxidized-type current confinement structure.
Japanese Unexamined Patent Application Publication No. 9-246660 discloses a method of stabilizing the transverse mode by disposing a lens structure comprising a circular diffraction grating within a laser. However, the process is complicated because a compound semiconductor layer should be re-grown. There are both technical and economical problems.
As described above, using the conventional technique, it is difficult to provide a surface light emitting semiconductor laser element that emits laser light in the single-peak transverse mode.